During the drilling of an oil or gas well or the like, situations are encountered wherein a component of the drill string becomes lodged in the borehole. It is, of course, necessary to dislodge this component of the drill string in order to continue the drilling operation. A rotary jarring tool is positioned in the drill string to allow the striking of blows to the drill string and the loosening of and dislodging of the stuck portion of the drill string. For example, rotary jarring tools are installed in fishing strings to enable the driller to strike heavy upward blows against an engaged fish to jar it loose from its stuck position. Rotary jarring tools are included in drill strings during testing, coring and wash-over operations to act as safeguards and to provide a system with which to loosen the drill string should it become stuck.
Rotary jarring tools include various types of restraining or detent mechanisms which hold the telescopic elements of the jarring tool in a closed position until sufficient upward pull is exerted to trip the restraining mechanism and allow the telescopic elements to rapidly move to their extended position. The force of the upward pull stretches the drill pipe. When the restraining mechanism trips, the upward surge of the drill pipe in returning to its normal length will allow a severe blow to be imparted to the drill string by the jarring tool.
Hydraulic jarring tools utilize a hydraulic working fluid and valve system to provide the tripping action. The jarring tool includes a seal system having upper and lower seal assemblies with the working fluid located therebetween. During the drilling operation, elevated temperatures are encountered which causes the physical properties of the working fluid to change. A need exists for a valving system that is reliable and provides closely repeating tripping action over a wide range of temperatures. The jar tool should be easy to manufacture and recocking of the jar tool should be simple and quick.